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Chemical degradation of Nafion ionomer at a catalyst interface of polymer electrolyte fuel cell by hydrogen and oxygen feeding in the anode
Microchemical Journal 106 (2013) 384–388
Contents lists available at SciVerse ScienceDirect
Microchemical Journal journal homepage: www.elsevier.com/locate/microc
Chemical degradation of Nafion ionomer at a catalyst interface of polymer electrolyte fuel cell by hydrogen and oxygen feeding in the anode Dodik Kurniawan a,⁎, Hayato Arai a, Shigeaki Morita b, Kuniyuki Kitagawa c a b c
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan Department of Engineering Science, Osaka Electro-Communication University, Neyagawa 572-8530, Japan Division of Energy Science, EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan
a r t i c l e
i n f o
Article history: Received 23 September 2012 Accepted 4 October 2012 Available online 9 October 2012 Keywords: Quantum chemical calculation Trifluoromethanesulfonate Chemical degradation Trace radical Hydrogen fluoride Bond dissociation energy
a b s t r a c t Chemical degradation products of a Nafion membrane on the catalyst interface of polymer electrolyte fuel cell (PEFC) by hydrogen and oxygen feeding into the anode were studied by high-performance liquid chromatography (HPLC). An acidic solution of pH 5.3 was obtained after the anode reaction and 5.0 × 10−6 mol·l−1 of hydrogen fluoride was detected by HPLC. Degradation mechanisms of the ionomer induced by trace radical species were simulated by means of quantum chemical calculations using a model side chain terminal of trifluoromethanesulfonate (TFMS). The results indicate that H radicals and OH radicals generated by the reaction on the interface induce the production of hydrogen fluoride, carbonyl fluoride and sulfonic acids. The bond dissociation energies (BDE) of C\S and C\F bond in TFMS are calculated to be 214.5 and 569.6 kJ·mol−1, respectively. It was clarified that the degradation of TFMS is affected not only by BDE but also by the behavior of fluorine atoms. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2þ
H2 O2 þ M
Durability is one of the most critical issues for commercialization of polymer electrolyte fuel cells (PEFC) [1]. Several macromolecular functions of proton-exchange membranes (PEM) have been required for stable and long-life operations of the fuel cells. Nafion, a perfluorosulfonic acid (PFSA) membrane developed by DuPont, has been widely applied as it is chemically stable, structurally durable and has a high proton conductivity when sufficiently hydrated [2–6]. Although many kinds of analogous PFSA based PEM have been developed, they do not adequately satisfy the demand for high performance fuel cell operations involving mechanical, thermal and electrochemical properties [7–10]. It has been believed that cumulative deterioration of the membrane is caused by trace radicals attacking the ionomer chain [11,12]. The generation of hydrogen radical, hydroxyl radical and degradation products by scission the ionomer chain is summarized as follows: H2 →2H ðvia Pt catalystÞ
ð1Þ
H þO2 ðdiffused through PEMÞ→ OOH
ð2Þ
H þ OOH→H2 O2 ðdiffused into PEMÞ
ð3Þ
⁎ Corresponding author. Tel.: +81 52 789 3913; fax: +81 52 789 3910. E-mail address: [email protected] (D. Kurniawan). 0026-265X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.microc.2012.10.004
2þ 2þ 3þ − Fe ; Cu ; etc: →M þ OH þ OH
R–CF2 –CF2 –COOH þ 2 OH→CO2 þ 2 HF þ R–CF2 –COOH:
ð4Þ ð5Þ
The performance of PEFC is known to be influenced by many parameters including the amount of trace radical and contamination of carbon monoxide (CO). Hori et al. investigated the lifetime properties of a single PEFC at 80 °C for up to 2500 h. Decrease in proton conductivity, increase in rate of hydrogen crossover and thinning membrane which directly related to PEFC power output were reported to be caused by the attack of the trace radical [13]. The adsorption of CO onto an anode catalyst also leads to the decrease in the output power of PEFC [14]. The trace amounts of CO in the hydrogen may be obtained by steam reforming of hydrocarbon fuels or a very expensive purification process should be applied [15]. Our previous work demonstrated that the adsorbed CO was removed by oxygen feeding into the anode for an oxidation reaction on the catalyst surface [14]. However, it is expected that the oxygen feeding directly into the anode causes chemical degradation of PEM. In the present study, mechanisms of chemical degradation of PEM on the catalyst interfaces induced by oxygen feeding into the anode of a PEFC will be discussed. Chemical species generated and/or degraded on a Pt/Nafion interface during hydrogen and oxygen feeding was analyzed by high-performance liquid chromatography (HPLC). In conjunction with this, quantum chemical calculations (QCC) based on density functional theory (DFT) was performed to investigate the chemical reactions of trifluoromethanesulfonate (TFMS). The calculation was
D. Kurniawan et al. / Microchemical Journal 106 (2013) 384–388
385
Thermocouple
Temperature Controller
Pt
Steel Plate
30 cm
Cell (Reactor)
Infrared Thermography Fig. 1. Schematic diagram of the experimental system equipped with infrared thermography.
made for a model side chain terminal of Nafion ionomer, with 1–3 hydrogen radicals (H•) and 1–3 hydroxyl radicals (•OH). Degradation products of TFMS and their dissociation energy (BDE) will be discussed in detail. 2. Experimental 2.1. Sample preparation A schematic diagram of the experimental system for this study is shown in Fig. 1. An infrared thermography (NEC Avio, InfRec Analyzer NS9500 Standard) was used to detect the temperature of the Pt/Nafion
interface inside the cell (reactor). The measured temperature was compared with a Pt-plated steel whose temperature was controlled. Fig. 2 shows the detailed illustrations of a laboratory-made reactor and its photograph. A Nafion (DuPont, N-117) membrane and the gas diffusion layer (GDL) were put between lower and upper parts of the reactor that was sealed using an O-ring. The carbon fiber based GDL that was integrated with the carbon-supported platinum catalyst of ca. 0.1 g·m−2 was used to effectively transport reactant gasses and electrons. Hydrogen and oxygen at flow rates of 6 and 3 ml·min−1, respectively were fed at standard temperature and pressure (STP) and exhausted at the upper side of the reactor. A liquid generated by the reactant gasses on the Pt/Nafion interface was collected after a 1 h
Fig. 2. Schematic design of the laboratory-made reactor. (a) Lower part, (b) upper part, (c) photograph of the reactor.
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D. Kurniawan et al. / Microchemical Journal 106 (2013) 384–388
3. Results and discussion 3.1. Temperature of Pt/Nafion interface Fig. 3 shows the temperature–time plot of the Pt surface due to hydrogen and oxygen feeding at flow rates of 6 and 3 ml·min−1, respectively. It was found that the temperature of Pt increased significantly in the first 10 s to reach approximately 60 °C and rose slowly to reach a saturation point, approximately 100 °C after 10 min due to heat of reaction at the surface. Surowiec et al. reported that Nafion is thermally stable up to 280 °C by evaluating with TG-DTA [16], hence thermal degradation did not occur in this condition. 3.2. pH value of generated solution
Fig. 3. Temperature–time plot of Pt surface due to hydrogen and oxygen feeding.
operation by disassembling the reactor and repeated several times to obtain the amount of 100 μl.
2.2. HPLC The acidity of the resulting liquid was measured using a pH meter (Oakton, pH Spear). A column for an anion analysis (Shodex, IC SI-50 4E) and an electric conductivity detector (Shimadzu, CDD-10Asp) were used for the HPLC. Aqueous solution of 3.2 mM Na2CO3 and 1.0 mM NaHCO3 were used as eluent. A sample volume of 100 μl was introduced into the column with the flow rate of the mobile phase at 0.7 ml·min −1.
2.3. QCC QCCs based on DFT were carried out with the Gaussian 03 program. All the computations were performed under conditions of a target temperature of 298 K and a pressure of 101.325 kPa (STP). The optimum structures and their BDEs were calculated with a B3LYP function and a 6-31++G(d,p) basis set. All the calculations refer to isolated molecules, e.g. gas phase and solvent influences were not taken into account. The optimized structure was calculated before the dissociation energy calculation.
The pH of the generated solution at the Pt/Nafion interface by hydrogen and oxygen feeding was ca. 5.3. This pH is slightly lower than the expected value for water at the fuel cell outlet, which should be similar to that of pure water in the presence of dissolved CO2 (pH ~ 5.7 for Milli-Q water left in air for 1 h) [17]. It is assumed that H+ is produced from chemical degradation reactions of the Nafion ionomer on the interface. From formula (5), H+ is expected to be potentially generated by hydrogen fluoride or carboxyl group as products of reaction between Nafion ionomer and H• and •OH. A concentration of HF in aqueous solution with pH 5.3 is calculated to be 5.0 × 10 −6 mol·l−1 assuming Ka is 6.8 × 10 −4 [18]. Lin et al. clearly found that pH value could be an important index of membrane degradation. The pH of generated water after 15 h and 100 h at 60 °C were ca. 4.3 and 3.5, respectively [19]. Assuming the pH is only derived from dissociation of HF in water, the initial concentrations of HF after 15 h and 100 h are 5.4 × 10−5 and 4.6 × 10 −4 mol·l−1, respectively. These values are much higher than that of our generated solution at the Pt/Nafion interface by hydrogen and oxygen feeding. Sethuraman et al. also found that temperature and operating time are the key factors to the progress of degradation by measuring the number of fluoride ion that dissociated from the Nafion membrane in Fenton solution test media [20]. 3.3. Detection of fluoride ion In order to identify ionic species in the liquid generated by the reactions, HPLC was applied. Fig. 4 shows the HPLC profile of liquid in the anode side generated by hydrogen and oxygen reaction. As a result, a peak of the fluoride ion was detected. It suggests that HF was formed by decomposition of Nafion and dissolved in water at the same time. 3.4. Calculation of BDE QCC was applied to unravel the degradation mechanisms of the Nafion ionomer via the scission mechanism and formation of HF as a product. BDEs of each bond in TFMS were calculated to seek
Table 1 BDEs and bond lengths at 298 K of TFMS calculated by DFT. Bond
Fig. 4. HPLC profile of solution in anode side generated by hydrogen and oxygen feeding.
C\S (CF3–SO3H) O\H (CF3SO2O–H) S_O (CF3HO2S_O) C\F (F–CF2SO3H)
B3LYP/6-31++G(d,p)
Experimental [18]
BDE (kJ·mol−1)
Bond length (nm)
BDE (kJ·mol−1)
Bond length (nm)
214.5 433.2 486.1 569.6
0.1917 0.0980 0.1473 0.1333
272 459 522 485
0.182 0.096 0.143 0.135
D. Kurniawan et al. / Microchemical Journal 106 (2013) 384–388
387
Fig. 5. Optimized structures of TFMS with 1–3 hydrogen radicals calculated by DFT.
weak bonds in the ionomer. BDE for a bond A\B which is broken through the reaction AB→A þ B
ð6Þ
is defined as the standard enthalpy change to dissociate the bond at a specific temperature, here at 298 K. That is, ΔHf 298 ¼ Hf 298 ðAÞ þ Hf 298 ðBÞ–Hf 298 ðABÞ
ð7Þ
where, ΔH is change in enthalpy, H(A) and H(B) are the enthalpy of reactants, and H(AB) is the enthalpy of product. Table 1 summarizes the BDEs and bond lengths of each bond in TFMS obtained by QCCs. Experimental data reported previously were also listed. In general, the shorter the bond length, the greater energy is required to dissociate the bond, whereas H\F and O\H bonds have high bond energies because of the high electronegativity of F and O atoms. The calculated BDEs indicate appropriate values compared with those estimated by X-ray diffraction [21]. The BDEs of C\F, S_O, O\H and C\S bonds in TFMS are calculated to be 569.6, 486.1, 433.2 and 214.5 kJ·mol −1, respectively, indicating that the C\S bond requires the smallest dissociation energy compared to the other bonds in the structure. However, the C\S bond becomes stronger in ionized PFSA or under high humidity condition [22]. 3.5. Degradation mechanism of Nafion Hereafter, the effects of H• and •OH on the chemical degradation of Nafion ionomer will be discussed using QCCs. Fig. 5 shows the optimized structures of TFMS with one to three hydrogen radicals (1–3 H•). The calculation results indicate that one H• attacks the CF3 group in the TFMS molecule and draws one F atom from the structure to yield HF. Whereas in the case of two or three H• attack, not only the C\F bond but also the C\S bond was dissociated from the structure. It is considered that H• is able to diffuse more freely to attack and dissociate the C\S bond due to the lower BDE followed by C\F bond breaking. Fig. 6 shows the optimized structures of TFMS with one to three hydroxyl radicals (1–3 •OH). The calculation results show that one or two •OH shows the protonation of the SO3H group in TFMS with the dissociation of the •OH. Whereas in
the case of three •OH, the formation of HF and dissociation of the C\F bond and C\S bond were calculated to occur. A different result with H•, the acidity of generated solution shows only in the presence of three •OH due to the formation of HF. Here, it has been clearly demonstrated that the degradation of TFMS is affected not only by BDE and bond length but also by the behavior of fluorine atoms. Fluorine efficiently shields the carbon skeleton from possible attacking of reagents, e.g. H• and •OH [23]. Once a perfluorination reaction proceeds, the carbon skeleton becomes sterically-protected by a sheath of fluorine atoms since the non-bonding electron pairs of fluorine inhibit further attack by incoming fluorine atoms [24]. Yu et al. reported that the energy barrier needed for H• to form HF from \CF2CF(CF3)OCF2CF2\ is 96.5 kJ·mol −1 and that for •OH to form HF from \CF2SO3H is 187.2 kJ·mol −1, calculated by B3LYP hybrid DFT functional with the Jaguar code and 6-311G** basis set [25]. In addition, it was revealed in the present study that chemical degradation of TFMS with formation of HF as small fragments due to 1–3 H• or 3 •OH attack takes place in the Pt/Nafion interface. The temperature of Pt was increased up to 100 °C or considered to exceed the energy barrier as a result of feeding hydrogen and oxygen simultaneously into the Pt/Nafion interface. 4. Conclusions The study of generated solution on the Pt/Nafion interface by feeding hydrogen and oxygen on the anode side to remove carbon monoxide is well suited to investigate the chemical degradation of PEFC. HPLC indicated that the solution consists of fluoride ion as product of degradation with a pH of 5.3 and a concentration of 5.0×10−6 mol·l−1. However, these values are slightly small compared with PEFC under the normal operating condition. DFT results calculated that 1–3 H• and 3 •OH induce the degradation of TFMS and formation of trace fragments, e.g., HF, CF2O and SO3H. It was found that the degradation of TFMS is affected not only by BDE but also by the behavior of fluorine atoms. Thus, the study of structural changes in TFMS is useful to unravel the degradation mechanism of Nafion, since the chemical degradation by trace radical is known to be the key parameter in controlling PEFC performance.
Fig. 6. Optimized structures of TFMS with 1–3 hydroxyl radicals calculated by DFT.
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References [1] M. Inaba, Durability of electrocatalysts in polymer electrolyte fuel cells, ECS Trans. 25 (1) (2009) 573–581. [2] C. Heitner-Wirguin, Recent advances in perfluorinated ionomer membranes: structure, properties and applications, J. Membr. Sci. 120 (1996) 1–33. [3] R.A. Lemons, Fuel cells for transportation, J. Power Sources 29 (1990) 251–264. [4] T.A. Zawodzinski, T.E. Springer, F. Uribe, S. Gottesfeld, Characterization of polymer electrolytes for fuel cell applications, Solid State Ionics 60 (1993) 199–211. [5] K. Strasser, Mobile fuel cell development at Siemens, J. Power Sources 37 (1992) 209–219. [6] K.D. Kreuer, On the development of proton conducting materials for technological applications, Solid State Ionics 97 (1997) 1–15. [7] T. Springer, T. Rockward, T. Zawodzinski, S. Gottesfeld, Model for polymer electrolyte fuel cell operation on reformate feed: effects of CO, H2 dilution, and high fuel utilization, J. Electrochem. Soc. 148 (1) (2001) A11–A23. [8] T. Loučka, The adsorption of sulphur and of simple organic substances on platinum electrodes, J. Electroanal. Chem. Interfacial Electrochem. 36 (2) (1972) 369–381. [9] X. Cheng, Z. Shi, N. Glass, L. Zhang, J. Zhang, D. Song, Z.-S. Liu, H. Wang, J. Shen, A review of PEM hydrogen fuel cell contamination: impacts, mechanisms, and mitigation, J. Power Sources 165 (2) (2007) 739–756. [10] R. Borupa, J. Daveya, F. Garzona, D. Wooda, M. Inbody, PEM fuel cell electrocatalyst durability measurements, J. Power Sources 163 (2006) 76–81. [11] A. Collier, H. Wang, X. Yuan, J. Zhang, D. Wilkinson, Degradation of polymer electrolyte membranes, Int. J. Hydrogen Energy 31 (13) (2006) 1838–1854. [12] D. Schiraldi, Perfluorinated polymer electrolyte membrane durability, Polym. Rev. 46 (3) (2006) 315–327. [13] J. Yu, T. Matsuura, Y. Yoshikawa, M.N. Islam, M. Hori, Lifetime behavior of a PEM fuel cell with low humidification of feed stream, Phys. Chem. Chem. Phys. 7 (2) (2005) 373–378.
[14] K. Kitagawa, S. Morita, S. Kimura, System of polymer electrolyte fuel cell, JP Patent 2010-257782, 2010. [15] K. Polychronopoulou, C.M. Kalamaras, A.M. Efstathiou, Ceria-based materials for hydrogen production via hydrocarbon steam reforming and water–gas shift reactions, Recent Pat. Mater. Sci. 4 (2) (2011) 1–24. [16] J. Surowiec, R. Bogoczek, Studies on thermal stability of the perfluorinated cation-exchange membrane Nafion-417, J. Therm. Anal. 33 (1988) 1097–1102. [17] A. Pozio, R. Silva, M. Francesco, L. Giorgi, Nafion degradation in PEFCs from end plate iron contamination, Electrochim. Acta 48 (11) (2003) 1543–1549. [18] T.L. Brown, H.E. LemMy, B.E. Bursten, Chemistry: The Central Science, 11th ed. Prentice-Hall, New York, 2008. [19] K.H. Hou, C.H. Lin, M.D. Ger, S.W. Shiah, H.M. Chou, Proc. 2011 International Conference on Environment Science and Engineering, vol. 8, IACSIT Press, Singapore, 2011. [20] V.A. Sethuraman, J.W. Weidner, A.T. Haug, L.V. Protsailo, Durability of perfluorosulfonic acid and hydrocarbon membranes: effect of humidity and temperature, J. Electrochem. Soc. 155 (2) (2008) B119–B124. [21] J.E. Huheey, T.L. Cottrell, The Strengths of Chemical Bonds, 2nd ed. Butterworths, London, 1958. [22] T. Tokumasu, I. Ogawa, M. Koyama, T. Ishimoto, A. Miyamoto, A DFT study of bond dissociation trends of perfluorosulfonic acid membrane, J. Electrochem. Soc. 158 (2) (2011) B175–B179. [23] P. Kirsch, Modern Fluoroorganic Chemistry: Synthesis, Reactivity, Applications, first ed. Wiley-VCH, Weinheim, 2004. [24] J. Hutchinson, G. Sandford, Elemental fluorine in organic chemistry, Organofluorine Chemistry, vol. 193, Springer, Berlin Heidelberg, 1997. [25] T.H. Yu, Y. Sha, W.G. Liu, B.V. Merinov, P. Shirvanian, W.A. Goddard, Mechanism for degradation of Nafion in PEM fuel cells from quantum mechanics calculations, J. Am. Chem. Soc. 133 (2011) 19857–19863.
Contents lists available at SciVerse ScienceDirect
Microchemical Journal journal homepage: www.elsevier.com/locate/microc
Chemical degradation of Nafion ionomer at a catalyst interface of polymer electrolyte fuel cell by hydrogen and oxygen feeding in the anode Dodik Kurniawan a,⁎, Hayato Arai a, Shigeaki Morita b, Kuniyuki Kitagawa c a b c
Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan Department of Engineering Science, Osaka Electro-Communication University, Neyagawa 572-8530, Japan Division of Energy Science, EcoTopia Science Institute, Nagoya University, Nagoya 464-8603, Japan
a r t i c l e
i n f o
Article history: Received 23 September 2012 Accepted 4 October 2012 Available online 9 October 2012 Keywords: Quantum chemical calculation Trifluoromethanesulfonate Chemical degradation Trace radical Hydrogen fluoride Bond dissociation energy
a b s t r a c t Chemical degradation products of a Nafion membrane on the catalyst interface of polymer electrolyte fuel cell (PEFC) by hydrogen and oxygen feeding into the anode were studied by high-performance liquid chromatography (HPLC). An acidic solution of pH 5.3 was obtained after the anode reaction and 5.0 × 10−6 mol·l−1 of hydrogen fluoride was detected by HPLC. Degradation mechanisms of the ionomer induced by trace radical species were simulated by means of quantum chemical calculations using a model side chain terminal of trifluoromethanesulfonate (TFMS). The results indicate that H radicals and OH radicals generated by the reaction on the interface induce the production of hydrogen fluoride, carbonyl fluoride and sulfonic acids. The bond dissociation energies (BDE) of C\S and C\F bond in TFMS are calculated to be 214.5 and 569.6 kJ·mol−1, respectively. It was clarified that the degradation of TFMS is affected not only by BDE but also by the behavior of fluorine atoms. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2þ
H2 O2 þ M
Durability is one of the most critical issues for commercialization of polymer electrolyte fuel cells (PEFC) [1]. Several macromolecular functions of proton-exchange membranes (PEM) have been required for stable and long-life operations of the fuel cells. Nafion, a perfluorosulfonic acid (PFSA) membrane developed by DuPont, has been widely applied as it is chemically stable, structurally durable and has a high proton conductivity when sufficiently hydrated [2–6]. Although many kinds of analogous PFSA based PEM have been developed, they do not adequately satisfy the demand for high performance fuel cell operations involving mechanical, thermal and electrochemical properties [7–10]. It has been believed that cumulative deterioration of the membrane is caused by trace radicals attacking the ionomer chain [11,12]. The generation of hydrogen radical, hydroxyl radical and degradation products by scission the ionomer chain is summarized as follows: H2 →2H ðvia Pt catalystÞ
ð1Þ
H þO2 ðdiffused through PEMÞ→ OOH
ð2Þ
H þ OOH→H2 O2 ðdiffused into PEMÞ
ð3Þ
⁎ Corresponding author. Tel.: +81 52 789 3913; fax: +81 52 789 3910. E-mail address: [email protected] (D. Kurniawan). 0026-265X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.microc.2012.10.004
2þ 2þ 3þ − Fe ; Cu ; etc: →M þ OH þ OH
R–CF2 –CF2 –COOH þ 2 OH→CO2 þ 2 HF þ R–CF2 –COOH:
ð4Þ ð5Þ
The performance of PEFC is known to be influenced by many parameters including the amount of trace radical and contamination of carbon monoxide (CO). Hori et al. investigated the lifetime properties of a single PEFC at 80 °C for up to 2500 h. Decrease in proton conductivity, increase in rate of hydrogen crossover and thinning membrane which directly related to PEFC power output were reported to be caused by the attack of the trace radical [13]. The adsorption of CO onto an anode catalyst also leads to the decrease in the output power of PEFC [14]. The trace amounts of CO in the hydrogen may be obtained by steam reforming of hydrocarbon fuels or a very expensive purification process should be applied [15]. Our previous work demonstrated that the adsorbed CO was removed by oxygen feeding into the anode for an oxidation reaction on the catalyst surface [14]. However, it is expected that the oxygen feeding directly into the anode causes chemical degradation of PEM. In the present study, mechanisms of chemical degradation of PEM on the catalyst interfaces induced by oxygen feeding into the anode of a PEFC will be discussed. Chemical species generated and/or degraded on a Pt/Nafion interface during hydrogen and oxygen feeding was analyzed by high-performance liquid chromatography (HPLC). In conjunction with this, quantum chemical calculations (QCC) based on density functional theory (DFT) was performed to investigate the chemical reactions of trifluoromethanesulfonate (TFMS). The calculation was
D. Kurniawan et al. / Microchemical Journal 106 (2013) 384–388
385
Thermocouple
Temperature Controller
Pt
Steel Plate
30 cm
Cell (Reactor)
Infrared Thermography Fig. 1. Schematic diagram of the experimental system equipped with infrared thermography.
made for a model side chain terminal of Nafion ionomer, with 1–3 hydrogen radicals (H•) and 1–3 hydroxyl radicals (•OH). Degradation products of TFMS and their dissociation energy (BDE) will be discussed in detail. 2. Experimental 2.1. Sample preparation A schematic diagram of the experimental system for this study is shown in Fig. 1. An infrared thermography (NEC Avio, InfRec Analyzer NS9500 Standard) was used to detect the temperature of the Pt/Nafion
interface inside the cell (reactor). The measured temperature was compared with a Pt-plated steel whose temperature was controlled. Fig. 2 shows the detailed illustrations of a laboratory-made reactor and its photograph. A Nafion (DuPont, N-117) membrane and the gas diffusion layer (GDL) were put between lower and upper parts of the reactor that was sealed using an O-ring. The carbon fiber based GDL that was integrated with the carbon-supported platinum catalyst of ca. 0.1 g·m−2 was used to effectively transport reactant gasses and electrons. Hydrogen and oxygen at flow rates of 6 and 3 ml·min−1, respectively were fed at standard temperature and pressure (STP) and exhausted at the upper side of the reactor. A liquid generated by the reactant gasses on the Pt/Nafion interface was collected after a 1 h
Fig. 2. Schematic design of the laboratory-made reactor. (a) Lower part, (b) upper part, (c) photograph of the reactor.
386
D. Kurniawan et al. / Microchemical Journal 106 (2013) 384–388
3. Results and discussion 3.1. Temperature of Pt/Nafion interface Fig. 3 shows the temperature–time plot of the Pt surface due to hydrogen and oxygen feeding at flow rates of 6 and 3 ml·min−1, respectively. It was found that the temperature of Pt increased significantly in the first 10 s to reach approximately 60 °C and rose slowly to reach a saturation point, approximately 100 °C after 10 min due to heat of reaction at the surface. Surowiec et al. reported that Nafion is thermally stable up to 280 °C by evaluating with TG-DTA [16], hence thermal degradation did not occur in this condition. 3.2. pH value of generated solution
Fig. 3. Temperature–time plot of Pt surface due to hydrogen and oxygen feeding.
operation by disassembling the reactor and repeated several times to obtain the amount of 100 μl.
2.2. HPLC The acidity of the resulting liquid was measured using a pH meter (Oakton, pH Spear). A column for an anion analysis (Shodex, IC SI-50 4E) and an electric conductivity detector (Shimadzu, CDD-10Asp) were used for the HPLC. Aqueous solution of 3.2 mM Na2CO3 and 1.0 mM NaHCO3 were used as eluent. A sample volume of 100 μl was introduced into the column with the flow rate of the mobile phase at 0.7 ml·min −1.
2.3. QCC QCCs based on DFT were carried out with the Gaussian 03 program. All the computations were performed under conditions of a target temperature of 298 K and a pressure of 101.325 kPa (STP). The optimum structures and their BDEs were calculated with a B3LYP function and a 6-31++G(d,p) basis set. All the calculations refer to isolated molecules, e.g. gas phase and solvent influences were not taken into account. The optimized structure was calculated before the dissociation energy calculation.
The pH of the generated solution at the Pt/Nafion interface by hydrogen and oxygen feeding was ca. 5.3. This pH is slightly lower than the expected value for water at the fuel cell outlet, which should be similar to that of pure water in the presence of dissolved CO2 (pH ~ 5.7 for Milli-Q water left in air for 1 h) [17]. It is assumed that H+ is produced from chemical degradation reactions of the Nafion ionomer on the interface. From formula (5), H+ is expected to be potentially generated by hydrogen fluoride or carboxyl group as products of reaction between Nafion ionomer and H• and •OH. A concentration of HF in aqueous solution with pH 5.3 is calculated to be 5.0 × 10 −6 mol·l−1 assuming Ka is 6.8 × 10 −4 [18]. Lin et al. clearly found that pH value could be an important index of membrane degradation. The pH of generated water after 15 h and 100 h at 60 °C were ca. 4.3 and 3.5, respectively [19]. Assuming the pH is only derived from dissociation of HF in water, the initial concentrations of HF after 15 h and 100 h are 5.4 × 10−5 and 4.6 × 10 −4 mol·l−1, respectively. These values are much higher than that of our generated solution at the Pt/Nafion interface by hydrogen and oxygen feeding. Sethuraman et al. also found that temperature and operating time are the key factors to the progress of degradation by measuring the number of fluoride ion that dissociated from the Nafion membrane in Fenton solution test media [20]. 3.3. Detection of fluoride ion In order to identify ionic species in the liquid generated by the reactions, HPLC was applied. Fig. 4 shows the HPLC profile of liquid in the anode side generated by hydrogen and oxygen reaction. As a result, a peak of the fluoride ion was detected. It suggests that HF was formed by decomposition of Nafion and dissolved in water at the same time. 3.4. Calculation of BDE QCC was applied to unravel the degradation mechanisms of the Nafion ionomer via the scission mechanism and formation of HF as a product. BDEs of each bond in TFMS were calculated to seek
Table 1 BDEs and bond lengths at 298 K of TFMS calculated by DFT. Bond
Fig. 4. HPLC profile of solution in anode side generated by hydrogen and oxygen feeding.
C\S (CF3–SO3H) O\H (CF3SO2O–H) S_O (CF3HO2S_O) C\F (F–CF2SO3H)
B3LYP/6-31++G(d,p)
Experimental [18]
BDE (kJ·mol−1)
Bond length (nm)
BDE (kJ·mol−1)
Bond length (nm)
214.5 433.2 486.1 569.6
0.1917 0.0980 0.1473 0.1333
272 459 522 485
0.182 0.096 0.143 0.135
D. Kurniawan et al. / Microchemical Journal 106 (2013) 384–388
387
Fig. 5. Optimized structures of TFMS with 1–3 hydrogen radicals calculated by DFT.
weak bonds in the ionomer. BDE for a bond A\B which is broken through the reaction AB→A þ B
ð6Þ
is defined as the standard enthalpy change to dissociate the bond at a specific temperature, here at 298 K. That is, ΔHf 298 ¼ Hf 298 ðAÞ þ Hf 298 ðBÞ–Hf 298 ðABÞ
ð7Þ
where, ΔH is change in enthalpy, H(A) and H(B) are the enthalpy of reactants, and H(AB) is the enthalpy of product. Table 1 summarizes the BDEs and bond lengths of each bond in TFMS obtained by QCCs. Experimental data reported previously were also listed. In general, the shorter the bond length, the greater energy is required to dissociate the bond, whereas H\F and O\H bonds have high bond energies because of the high electronegativity of F and O atoms. The calculated BDEs indicate appropriate values compared with those estimated by X-ray diffraction [21]. The BDEs of C\F, S_O, O\H and C\S bonds in TFMS are calculated to be 569.6, 486.1, 433.2 and 214.5 kJ·mol −1, respectively, indicating that the C\S bond requires the smallest dissociation energy compared to the other bonds in the structure. However, the C\S bond becomes stronger in ionized PFSA or under high humidity condition [22]. 3.5. Degradation mechanism of Nafion Hereafter, the effects of H• and •OH on the chemical degradation of Nafion ionomer will be discussed using QCCs. Fig. 5 shows the optimized structures of TFMS with one to three hydrogen radicals (1–3 H•). The calculation results indicate that one H• attacks the CF3 group in the TFMS molecule and draws one F atom from the structure to yield HF. Whereas in the case of two or three H• attack, not only the C\F bond but also the C\S bond was dissociated from the structure. It is considered that H• is able to diffuse more freely to attack and dissociate the C\S bond due to the lower BDE followed by C\F bond breaking. Fig. 6 shows the optimized structures of TFMS with one to three hydroxyl radicals (1–3 •OH). The calculation results show that one or two •OH shows the protonation of the SO3H group in TFMS with the dissociation of the •OH. Whereas in
the case of three •OH, the formation of HF and dissociation of the C\F bond and C\S bond were calculated to occur. A different result with H•, the acidity of generated solution shows only in the presence of three •OH due to the formation of HF. Here, it has been clearly demonstrated that the degradation of TFMS is affected not only by BDE and bond length but also by the behavior of fluorine atoms. Fluorine efficiently shields the carbon skeleton from possible attacking of reagents, e.g. H• and •OH [23]. Once a perfluorination reaction proceeds, the carbon skeleton becomes sterically-protected by a sheath of fluorine atoms since the non-bonding electron pairs of fluorine inhibit further attack by incoming fluorine atoms [24]. Yu et al. reported that the energy barrier needed for H• to form HF from \CF2CF(CF3)OCF2CF2\ is 96.5 kJ·mol −1 and that for •OH to form HF from \CF2SO3H is 187.2 kJ·mol −1, calculated by B3LYP hybrid DFT functional with the Jaguar code and 6-311G** basis set [25]. In addition, it was revealed in the present study that chemical degradation of TFMS with formation of HF as small fragments due to 1–3 H• or 3 •OH attack takes place in the Pt/Nafion interface. The temperature of Pt was increased up to 100 °C or considered to exceed the energy barrier as a result of feeding hydrogen and oxygen simultaneously into the Pt/Nafion interface. 4. Conclusions The study of generated solution on the Pt/Nafion interface by feeding hydrogen and oxygen on the anode side to remove carbon monoxide is well suited to investigate the chemical degradation of PEFC. HPLC indicated that the solution consists of fluoride ion as product of degradation with a pH of 5.3 and a concentration of 5.0×10−6 mol·l−1. However, these values are slightly small compared with PEFC under the normal operating condition. DFT results calculated that 1–3 H• and 3 •OH induce the degradation of TFMS and formation of trace fragments, e.g., HF, CF2O and SO3H. It was found that the degradation of TFMS is affected not only by BDE but also by the behavior of fluorine atoms. Thus, the study of structural changes in TFMS is useful to unravel the degradation mechanism of Nafion, since the chemical degradation by trace radical is known to be the key parameter in controlling PEFC performance.
Fig. 6. Optimized structures of TFMS with 1–3 hydroxyl radicals calculated by DFT.
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